Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising. Typically, a transmission has a housing mounted to the vehicle structure, an input shaft driven by an engine crankshaft, and an output shaft driving the vehicle wheels, often via a differential assembly which permits the left and right wheel to rotate at slightly different speeds as the vehicle turns.
A common type of automatic transmission utilizes a collection of clutches and brakes. Various subsets of the clutches and brakes are engaged to establish the various speed ratios. A common type of clutch utilizes a clutch pack having separator plates splined to a housing and interleaved with friction plates splined to a rotating shell. When the separator plates and the friction plates are forced together, torque may be transmitted between the housing and the shell. Typically, a separator plate on one end of the clutch pack, called a reaction plate, is axially held to the housing. A piston applies axial force to a separator plate on the opposite end of the clutch pack, called a pressure plate, compressing the clutch pack. The piston force is generated by supplying pressurized fluid to a chamber between the housing and the piston. For a brake, the housing may be integrated into the transmission case. For a clutch, the housing rotates. As the pressurized fluid flows from the stationary transmission case to the rotating housing, it may need to cross one or more interfaces between components rotating at different speeds. At each interface, seals direct the flow from an opening in one component into an opening in the interfacing component.
In a rear wheel drive vehicle, the engine and transmission are commonly mounted along a longitudinal axis of the vehicle, perpendicular to the axis about which the wheels rotate. Power from a transmission output shaft is conveyed by a driveshaft to a differential assembly which changes the axis of rotation, multiplies the torque by a final drive ratio, and permits the wheels to rotate at slightly different speeds. In a front wheel drive vehicle, the engine and transmission are commonly mounted transversely, such that the engine crankshaft rotates about an axis parallel to the axis about which the wheels rotate. In a front wheel drive vehicle, it is common to combine the transmission and the differential into a single unit called a transaxle.
Several vehicle design trends combine to limit the space available for a transaxle. A desire to limit the frontal area to reduce drag and improve styling tends to reduce the engine compartment width. Switching from engines with the cylinders arranged in a V pattern to engines with the cylinders inline increases the length of the engine. At the same time, the number of distinct transmission ratios has been tending to increase, increasing the number of parts required in the transmission. Consequently, transaxles must be designed with some components radially within other components in order to minimize the total axial length.